1. Field of the Invention (Technical Field)
The present invention relates to the field of internal combustion engine exhaust gas recirculation (EGR) for emissions improvement and increased operating efficiency, including increased fuel economy. More particularly, the invention provides a system and method for dual path EGR, utilizing a high pressure EGR loop, primarily for use under mid and high load engine conditions, and a low pressure EGR loop, combining turbocharger compressed exhaust gas and fresh air, primarily for use under low load engine conditions.
2. Description of Related Art
EGR is a known method for reducing NOx emissions in internal combustion engines, particularly diesel engines. For effective use, an EGR system must be matched to the engine load setting and environmental conditions. High pressure loop EGR, providing a loop from the exhaust manifold of the engine to the intake manifold, optionally with an EGR cooler, works well for high power or load settings and modest EGR rates. However, at low load settings the comparatively high EGR rates result in undesirably high back pressures, resulting in a significant fuel consumption penalty.
Low pressure loop EGR typically takes exhaust gas from the exhaust, downstream of particulate traps and other emissions control devices, and injects the exhaust gas into the compressor, delivering compressed exhaust gas, typically mixed with fresh air, to the intake manifold of the engine. Low pressure loop EGR works well at low power or load settings. At high EGR rates low pressure loop EGR typically incurs some operational penalties, in that the balance of EGR to fresh air is such as to not provide optimal emissions control. However, at high power or load settings low pressure loop EGR incurs a very large fuel penalty, due to the need for flow ranges outside the capabilities of turbine-driven compressors, a poor turbine-to-compressor flow match and excessive heat loads.
It is also known that environmental conditions can affect the operation and efficiency of EGR systems. Specifically, in cold weather condensation is a significant issue with EGR systems, particularly high pressure loop EGR. Condensation in cold weather, typically caused by operation of the EGR cooler, results in introduction of water into the intake manifold of the engine, causing production of acids, including sulfuric and nitric acids, soot formation, and the like, all of which degrade engine life and performance.
It is known to introduce only fresh air to the inlet of an engine, typically air compressed by use of a turbine compressor. However, with use of fresh air only the benefits, including the emission control benefits, resulting from use of EGR are lost. In some systems, high pressure loop EGR is employed in combination with compressed fresh air. However, these systems still result in undesirable emission levels at low power or load settings, and are susceptible to decreased efficiency and fuel consumption penalties.
It is, therefore, desirable to provide an EGR system, and methods for such systems, which provide optimal fuel and emissions controls over a wide range of power or load settings. In many applications, including diesel engines for vehicles, the power or load setting varies over a wide range, from very low loads while the vehicle is at idle to very high loads while the vehicle is accelerating or climbing an incline. In such applications a wide range of environmental operating conditions are typically encountered, with ambient air temperatures ranging from well below 0° C. to over 40° C. It is further desirable to provide an EGR system, and methods for such systems, which provide the benefits associated with high pressure EGR loop systems, without incurring the penalties associated with such systems, and which further provide the benefits associated with low pressure EGR loop systems, again without incurring the penalties associated with such systems.